PSCC on 460V Split Phase Supply

[Blissof89]

Hi all.

Been doing a bit of work on a pub out in the sticks with a 460V split phase Supply. When tried to test phase - phase PSCC my tester didn't play along. I was going to use the double p - n rule of thumb but then I got thinking, does this still apply to a 460v supply?

 

[Lucien Nunes]

If your tester was able to give you readings for the loop impedance line-line, then surely you can just divide 460 by that impedance to get the PSCC. AFAIK the tester measures the impedance, decides whether the voltage it's connected to is within spec for 230 or 400V and calculates the PSCC based on that nominal voltage. Presumably when presented with 460V it doesn't recognise what the intended supply voltage is, so won't calculate for you.

 

[westward10]

Most testers will have a voltage tolerance and I suspect 460v was above it so it defaulted to a safe mode to prevent damage.

 

[Toneyz]

If your tester was able to give you readings for the loop impedance line-line, then surely you can just divide 460 by that impedance to get the PSCC. AFAIK the tester measures the impedance, decides whether the voltage it's connected to is within spec for 230 or 400V and calculates the PSCC based on that nominal voltage. Presumably when presented with 460V it doesn't recognise what the intended supply voltage is, so won't calculate for you.

What tester are you using?

 

[gaz101]

Interesting to see how many sparks dont know about split phase done 2 new installs with it last year in devon

 

[Lucien Nunes]

I suspect 460v was above it so it defaulted to a safe mode to prevent damage.

The OP mentions it gave a reading for impedance between the lines, so it was apparently OK with measuring at 460V. But as I expect with all MFT's, it doesn't 'know' that this was a valid nominal voltage so it couldn't calculate the PSCC from the impedance as it would at 400V. The OP simply has to calculate it himself.

 

[Deleted member 26818]

Do they make testers that can measure 3 phase PSCC now?
I’ve always just measured each phase to earth/neutral and added the two highest values together.

 

[Lucien Nunes]

I can't recall having a tester that didn't, all the way back to my first MFT (Megger MF200? or did I make that number up? square grey box that did 4-wire Ra tests too, but before non-trip RCD tests).

Unless I'm misunderstanding you. They don't measure the PFCC, just calculate it from loop impedance, so if the unit will test Zp-p then that counts too.

 

[davesparks]

The sub stations from center point would show 415 , from their to a farm or a commercal buildings and from thier to motors so how could the building you at testing on come up 460v ,you better ring the DNO to clarify what voltage you have got .

That's for a three phase supply, this is not a three phase supply, this is a split phase supply which has 460V between the lines and 230V to neutral from each line.

 

[buzzlightyear]

a standard 3 phase 230/400 volt D/B cant be used at the origin because the 460 volt supply exceeds the 400 volt rating of the equipment.

 

[Lucien Nunes]

But 690V gear is fine, and in the past most industrial gear was rated at 500V which was OK too.

 

[SparkySy]

We have split phase on the farm we are on it was originally for the dairy and feed mill, the dairy is long gone but the feed mill gets used regularly for the calf's and winter feed mix.
I have always done the same as spin on testing, took the highest reading off both phases to neutral or earth and added them together!
Not entirely sure if that was the correct method but here to learn as well as comment!
Sy

 

[Risteard]

Do they make testers that can measure 3 phase PSCC now?

Both my MFT1741 and my much older MFT1553 are capable of directly measuring between phases.

It's generally only the lower-end units now which are incapable of connection between phases.

 

[Blissof89]

If your tester was able to give you readings for the loop impedance line-line, then surely you can just divide 460 by that impedance to get the PSCC. AFAIK the tester measures the impedance, decides whether the voltage it's connected to is within spec for 230 or 400V and calculates the PSCC based on that nominal voltage. Presumably when presented with 460V it doesn't recognise what the intended supply voltage is, so won't calculate for you.

I was thinking something along this line just wasn't sure on it, thanks for clarifying

So PSCC= 460/whatever that impedance value was, I don't think I wrote it down

 

[GeorgeCooke]

Got a split phase supply like this on a holiday site on Isle of Wight. It is actually 480volts, big warning signs on all the switchgear in the intake room. Confirmed by measuring.

 

[freddo]

Interesting to see how many sparks dont know about split phase done 2 new installs with it last year in devon

Same, there seems to be loads in Devon!

Do they make testers that can measure 3 phase PSCC now?
I’ve always just measured each phase to earth/neutral and added the two highest values together.

Most modern testers will, mine will do all tests on supplies between 55V and 500V AC.